1. Field of the Invention
This invention relates to breathing demand regulators and is particularly concerned with breathing demand regulators having facility for mixing air with oxygen enriched breathing gas in aircraft breathing systems.
2. Description of the Prior Art
It is known practice to supply 100% oxygen gas for aircraft aircrew breathing purposes from a liquid oxygen (LOX) system which converts liquid oxygen to gaseous oxygen in a converter. The gaseous oxygen is delivered to an aircrew breathing mask byway of a regulator having a valve member that opens to allow oxygen to flow to the mask in response to the breathing demands of the aircrew member.
At lower altitudes, generally below 9000 m (30000 ft), the aircrew member may become over oxygenated if 100% oxygen gas is supplied for breathing purposes. It is usual, therefore, for the regulator to include a facility for entrainment of ambient air to reduce the content of oxygen in the breathing gas delivered to the breathing mask. Disclosures of such breathing regulators are to be found, for example, in U.S. Pat. No. 2,384,669; GB-A-630,740; and U.S. Pat. No. 4,928,682.
Whilst aircraft having LOX systems continue in operation at the present time, it is generally the practice in new aircraft designs to provide an on-board oxygen generating system (OBOGS) in which oxygen-enriched breathable gas is derived from a molecular sieve oxygen concentrator (MSOC). The MSOC comprises two or more molecular sieve beds which retain nitrogen in air supplied to the beds so that product gas enriched in oxygen is delivered from the MSOC. Each sieve bed is cycled between an on-stream phase in which air is supplied to the bed and oxygen-enriched product gas is delivered from the bed, and an off-stream phase in which the bed is vented to ambient and back-flushed with oxygen-enriched product gas to cleanse it of retained nitrogen ready for the next on-stream phase. Cycling of the beds may be controlled to produce product gas of maximum oxygen concentration, usually between 90% and 95% oxygen, or the beds may be cycled to produce product gas enriched with oxygen to a concentration appropriate to maintaining oxygen partial pressure in the product gas at a constant value substantially equivalent to sea level partial pressure of oxygen in air, irrespective of the altitude of the aircraft. Whilst in some aircraft aircrew breathing systems employing an OBOGS for supplying breathable gas the latter method of control is used, in other systems preference is for the former method of control. In either case product gas is delivered to an aircrew member by way of a breathing demand regulator which in the case where the product gas is of maximum oxygen concentration has provision for entrainment of air to reduce the oxygen concentration at lower altitudes.
There is now a need for a breathing regulator capable of operating with both LOX and OBOGS systems. This imposes a requirement for a breathing regulator capable of operating over a wide range of inlet pressure and, in particular, down to very low inlet pressures, for example, 70 kPa (10 psi), under certain aircraft operating conditions.
Breathing demand regulators, by virtue of having to present minimum resistance to breathing efforts of an aircrew member, should critically balanced mechanisms and as such are sensitive to variations in pressure loading of a main demand valve which results from variations in both upstream and downstream conditions.
In its simplest form, in a breathing demand regulator using 100% OBOGS product gas, the mechanism consists of a sensing member, usually a diaphragm, which is linked either mechanically or pneumatically to the demand valve. The loading of this mechanism from a combination of pneumatic and spring forces, ensures that under zero demand conditions the demand valve remains closed but with minimum reduction in pressure on one side of the sensing diaphragm resulting from inhalation by the aircrew member, the demand valve is opened and flow is delivered to satisfy demand.
All breathing demand regulators are subject to variation in upstream conditions (supply pressure variations) but where possible these are minimised by utilising a pressure reducing valve or pressure limiting valve either upstream of the regulator or integral with the regulator. However, some systems do not afford this facility and the demand mechanism has to cope with wide variations in supply pressure, typically in regulators which are required to operate with an OBOGS and, in some cases, also to be compatible with LOX systems.
A breathing demand regulator disclosed in EP-A-0,078,644 (Normalair-Garrett) overcomes both the problem of large variations in supply conditions and the problem of demand valve operation at the lower range of oxygen-enriched breathable gas pressure available from a MSOC, particularly at the lower end towards 70 kPa (10 psi). This regulator embodies a diaphragm arranged for sensing breathing demand and actuating, via a lever, a demand valve having a Valve head carried by a stem projected by a spool member. Opposed surface areas of the valve head and spool member are equal so that the valve is balanced by the pressure of oxygen-enriched breathing gas entering an inlet disposed therebetween and variations in upstream pressure are of no effect.
U.S. Pat. No. 4,928,682 (Normalair-Garrett) discloses a modified form of the aforementioned breathing demand regulator (EP-A-0,078,644) having facility for entrainment of ambient air for mixing with maximum concentration oxygen-enriched product gas supplied to the regulator whereby breathing gas of reduced oxygen enrichment may be supplied to an aircrew member. The inventive feature of this disclosure relates to control of an injector nozzle bypass whereby in one mode of regulator operation an ambient air inlet control valve is closed and an injector bypass is open so that undiluted MSOC product gas is delivered to a regulator outlet, and in another mode of operation the air inlet control valve is open and the injector bypass is closed so that MSOC product gas flows through the injector nozzle to induce ambient air to enter the regulator and mix with the MSOC product gas whereby breathable gas of diluted oxygen concentration is delivered to the regulator outlet.
For efficiency of operation in the airmix mode the modified regulator depends upon driving pressure behind the injector nozzle to create the necessary pressure drop across the nozzle and hence the energy to entrain air through the air inlet control valve. The driving pressure is a function of flow and nozzle size. The flow varies from zero to a maximum, depending upon the size of the demand made on the regulator, therefore the efficiency varies from zero to some value related to the nozzle size.
The nozzle size invariably has to be a compromise:
i. it has to be of a size which will provide the required efficiency in terms of air/oxygen mixture over the full range of breathing flows; PA1 ii. it has to be sufficiently large to pass the minimum amount of MSOC oxygen enriched product gas at peak flow demands; PA1 iii. in order to meet i. it has to remain seated for most of the demand flow range but may have to relieve at the top end of the flow range to meet ii; PA1 iv. it has to be as large as possible to reduce the tendency towards oscillatory activity; PA1 v. it has to be as large as possible to reduce the downstream feedback onto the demand valve which has a detrimental effect on breathing.
There is a direct conflict between the requirements to meet i. and those to meet ii. to v. inclusive. Furthermore, if the regulator incorporates a flow indicator which relies for its operation on injector driving pressure, this imposes yet another function on the injector system since the nozzle then has to be small enough to generate a high enough pressure to operate the flow indicator mechanism within the required limits.
Development of an airmix regulator based on the disclosure of U.S. Pat. No. 4,928,682 has highlighted all the difficulties previously experienced with the development of airmix regulators and, unfortunately, the point made at v., above has shown itself to be more of a problem than was originally expected, particularly in a system where a high boost characteristic is required to overcome high system back pressure. Pressure build-up downstream of the demand valve resulting from the flow restricting effect of the injector nozzle causes a pressure feedback onto the head of the valve tending to force it closed. To overcome this effect the aircrew member must suck harder when demanding breathing gas so that breathing effort becomes more tiresome, or additional boost must be built into the regulator by changing the configuration of the demand sensing region in the outlet of the regulator which makes it more difficult to control overshoot of the demand valve on opening.